Many wideband digital communication systems utilize Orthogonal Frequency-Division Multiplexing (OFDM) as the modulation scheme. OFDM uses a large number of closely spaced orthogonal sub-carriers, each of which is modulated with a conventional modulation scheme at a low symbol rate. In OFDM-based wide area broadcasting, a plurality of receivers can simultaneously receive signals from several spatially-dispersed transmitters. In these systems, OFDM is typically combined with other forms of space diversity, such as antenna arrays and multiple input/multiple output (MIMO) channels. One example of such a system is the IEEE 802.11n Wireless LAN (Local Area Network) standard.
The Modulation and Coding Scheme (MCS) is a value that determines the modulation, coding and number of spatial channels in MIMO-OFDM systems. It is a compact representation that is carried in the high throughput signal field of the channels. Fast and accurate selection of the MCS level and spatial transmission rate is crucial for exploiting the potentially high spectral efficiency of adaptive wireless MIMO-OFDM systems. Measurements for MCS level selection and/or the decision to switch to a particular spatial rate for transmission is typically made at the receiver end of the wireless link, and is then conveyed back to the transmitter side via the low rate feedback path. This process can also be performed at the transmitter based on certain known carrier information.
In many practical systems, the number of effective transmitter antennas is limited to only two. For these systems, the spatial rate selection mechanism involves selecting either diversity coding (spatial rate 1), or spatial multiplexing (spatial rate 2) as the method of MIMO transmission.
Present methods of switching between the diversity coding method and spatial multiplexing method generally base the spatial rate selection on the comparative values of the minimum Euclidean distances for diversity coding and spatial multiplexing. For a fixed spectral efficiency, the scheme which has the higher minimum Euclidean distance is usually selected for transmission. The calculation of such distances can be a rather complex process. Moreover, although such an optimization criterion may minimize the probability of error under the assumption of the fixed throughput, in systems with different possible throughput channels, such as multiple MCS-level systems, such a method may often underestimate the throughput rate in order to minimize the probability of error. Consequently, such systems may operate with less performance than is possible. Such prior art systems utilize a different optimization criteria than is used by practical link adaptation algorithms. The goal of these link adaptation algorithms is typically to maximize throughput under a fixed target probability of error, or a maximum allowed expected error rate. Present rate selection methods for MIMO-OFDM systems, however, generally do not allow for maximum throughput to be factored into the rate selection process.